Something can be priceless despite reproductions of it being available for zero cost.
The question you don't ask is: What is being reproduced? If you're talking about just a band playing a song, then maybe I'd agree with you. I don't have an enormous problem with people listening to bootleg recordings of live bands. As you say there, those are recordings that stemmed from a concert that already made its money.
But when the music is recorded, when you hire audio engineers to get as good a sound as possible, when you slave over making an absolutely perfect recording, then you've created an object whose initial worth is determined by the exclusivity of the releases. When you spend lots of effort to release a CD, you're doing it because you expect the deal is going to be: "If you want these high quality recordings, you pay me a little bit of money."
When that gets released for free, all the efforts that went into making that particular sound are rendered valueless because the band's expected return suddenly isn't happening.